It is known to produce isomeric mixtures of dichlorobenzene by contacting gaseous chlorine with benzene or monochlorobenzene in the liquid phase in the presence of a metal halide catalyst, e.g., ferric chloride, aluminum chloride, antimony chloride and the like. However, in this process, the maximum molar ratio or para-dichlorobenzene isomer to orthodichlorobenzene isomer is only about 1.5 to 1. Since the para-dichlorobenzene isomer is the more valuable product it is desirable to increase the proportion of para- to ortho isomer.
In order to effect this desirable increase in proportion of para- to ortho isomers in the product dichlorobenzene, the prior art has employed elemental sulfur or sulfur-containing compounds as co-catalysts along with the metal halide as described, for example, in U.S. Pat. No. 3,226,447 and Australian Pat. No. 223,024. Since, however, hydrochloric acid is often recovered as a salable by-product, care must be taken to avoid sulfur contamination of the hydrochloric acid as well as sulfur contamination of the product dichlorobenzene.